Active medical devices are generally designed to enable a bidirectional data exchange with a remote “programmer,” which is an external device, for a variety of reasons, e.g., to verify the configuration of the device, to read information the device has recorded, to post information to the device, or to update the internal software of the device.
Techniques for monitoring patients at home (home monitoring) have been proposed that implement a remote interrogation device disposed adjacent to the implanted patient, which is periodically activated, e.g., daily, to download data collected by an implanted device and transmission for analysis to a remote surveillance site. Data exchange between an implantable device and the programmer or home monitoring device (hereinafter, collectively an “external device”) is generally performed by telemetry, that is to say, by a communication technique for remote transmission of information, without galvanic contact.
A common telemetry technique uses inductive coupling between coils located inside the implanted device and those in the external device, which technique is known as “induction telemetry”. The induction telemetry technique has the disadvantage, because of the very limited range of such a coupling, to require the use of a “telemetry head” connected to the external device and containing a coil that an operator places in the vicinity of the site in which the device is implanted.
It was recently proposed to implement an alternative technique for non-galvanic coupling, using an electromagnetic wave generated by and between complementary transmitter/receiver circuits operating in the radiofrequency (RF) domain, typically in the range of frequencies of several hundred megahertz. This technique, known as “RF telemetry” allows programming or interrogating implants at distances greater than 3 m, and therefore permits the exchange of information without manipulation of a telemetry head, and indeed even without external operator intervention.
An active medical device implementing RF telemetry is described for example in EP1862195A1 and its US counterpart U.S. Pat. No. 7,663,451 (both assigned to Sorin CRM S.A.S., previously known as ELA Medical).
Power Consumption by RF telementary circuits is a crucial aspect of this RF technique, especially with regard to an implanted device. Indeed, the implanted devices use for energy sources a battery whose energy capacity is limited (about 1 Ah). Without RF telemetry, the average consumption of an implanted device is about 20 μW, which provides autonomy (i.e., a useful life under normal circumstances) of about a dozen years. However, upon activation of a RF telemetry function, an implanted device typically consumes, with current technologies, a dozen milliwatts, about 500 to 1000 times more than its average consumption for the usual functions of cardiac sensing and pacing.
On the other hand, it is known that the environment surrounding the patient has more and more radio frequency interferences that could inadvertently and adversely wake up the RF telemetry circuit of the implanted device and therefore unnecessarily discharge the battery of the implanted device.
Occasional use of the RF telemetry once every three or six months, for example, in connection with follow-up visits to the cardiologist, does not significantly degrade the autonomy of the implanted device. However, daily use to communicate with a home monitoring device can significantly reduce the lifetime of the implanted device. Indeed, if for example a quarter of the capacity of the battery is assigned to the RF telemetry functions, this capacity provides only 50 hours of continuous RF communication throughout the useful life of the implanted device (10 years), or a daily use of less than 50 seconds.
Knowing that the current RF telemetry systems use communication channels of bandwidth limited to 300 kHz with a data rate of about 100-200 kbps (kilobits per second), and that the electrogram (“EGM”) signals are sampled on 10 bits and 500 times per second, a transmission duration of 50 seconds per day allows transmission to the external device (taking into account the overload introduced by the communication protocol) of only a maximum of 8 minutes recording of two EGM signal channels.
There is therefore a need for an intensive use of a remote RF telemetry system that is efficient in terms of energy use without penalizing the autonomy of the implanted device.